[KnownBits] Add blsi and blsmsk

[foad]

Upload the right patch

[foad]

@goldstein.w.n is doing this in D142519 now.

llvm/include/llvm/Support/KnownBits.h

public:
   KnownBits byteSwap() const {
     return KnownBits(Zero.byteSwap(), One.byteSwap());
   }
 
   KnownBits reverseBits() const {
     return KnownBits(Zero.reverseBits(), One.reverseBits());
   }
 
-  /// Compute known bits for X & -X.
+  /// Compute known bits for X & -X. The name comes from the X86 BMI instruction
+  /// BLSI.
   KnownBits blsi() const;
 
-  /// Compute known bits for X ^ (X - 1).
+  /// Compute known bits for X ^ (X - 1). The name comes from the X86 BMI
+  /// instruction BLSMSK.
   KnownBits blsmsk() const;
 
   bool operator==(const KnownBits &Other) const {
     return Zero == Other.Zero && One == Other.One;
   }
 
   bool operator!=(const KnownBits &Other) const { return !(*this == Other); }
 

llvm/lib/Support/KnownBits.cpp

KnownBits &KnownBits::operator^=(const KnownBits &RHS) {
   APInt Z = (Zero & RHS.Zero) | (One & RHS.One);
   // Result bit is 1 if one operand bit is 0 and the other is 1.
   One = (Zero & RHS.One) | (One & RHS.Zero);
   Zero = std::move(Z);
   return *this;
 }
 
 KnownBits KnownBits::blsi() const {
-  KnownBits Known(getBitWidth());
-  unsigned Min = countMinTrailingZeros();
+  unsigned BitWidth = getBitWidth();
+  KnownBits Known(Zero, APInt(BitWidth, 0));
   unsigned Max = countMaxTrailingZeros();
-  Known.Zero.setLowBits(Min);
-  Known.Zero.setBitsFrom(Max + 0);
+  Known.Zero.setBitsFrom(std::min(Max + 1, BitWidth));
+  unsigned Min = countMinTrailingZeros();
+  if (Max == Min && Max < BitWidth)
+    Known.One.setBit(Max);
   return Known;
 }
 
 KnownBits KnownBits::blsmsk() const {
-  KnownBits Known(getBitWidth());
-  unsigned Min = countMinTrailingZeros();
+  unsigned BitWidth = getBitWidth();
+  KnownBits Known(BitWidth);
   unsigned Max = countMaxTrailingZeros();
-  Known.One.setLowBits(Min + 1);
-  Known.Zero.setBitsFrom(Max + 0);
+  Known.Zero.setBitsFrom(std::min(Max + 1, BitWidth));
+  unsigned Min = countMinTrailingZeros();
+  Known.One.setLowBits(std::min(Min + 1, BitWidth));
   return Known;
 }
 
 void KnownBits::print(raw_ostream &OS) const {
   OS << "{Zero=" << Zero << ", One=" << One << "}";
 }
 void KnownBits::dump() const {
   print(dbgs());
   dbgs() << "\n";
 }